Interest in the use of controllable fluids revived in the 1980's as activity in the area of controllable dampers increased. Most of the resurgent activity has occurred relative to ER dampers and associated fluids. While interest and development of ER fluid devices continues performance of such systems have been disappointing from three standpoints:
1) the damping forces that can be generated by an ER fluid device are limited due to the relatively low yield strengths of the available fluids;
2) ER fluids are susceptible to contamination which significantly degrades performance; and,
3) the strong electric fields required by ER fluids necessitate complicated and expensive high-voltage power supplies and complex control systems.
Faced with these performance restrictions and searching for a technological breakthrough to overcome them, Applicants turned to MR fluids with renewed interest and sought to optimize systems employing them. MR fluids inherently have higher yield strengths and are, therefore, capable of generating greater damping forces. Further, contamination does not pose the performance degradation threat for MR fluids that it does for ER fluids. Still further, MR fluids are activated by magnetic fields which are easily produced by simple, low-voltage electromagnetic coils.
It is therefore among the objects of the present invention to provide an MR damper with improved performance characteristics. Enhancements include:
defining dimensional/operational relationships which provide improved performance; PA1 devising piston designs in which the flow path for the magnetic flux is entirely contained within the piston itself; PA1 providing an improved twin-tube cylinder design capable of use (with some modification) with either the self-contained or spool piston; PA1 significantly reducing or eliminating MR fluid losses from the damper; PA1 providing an improved fluid valve for controlling the flow of the MR fluid to produce the desired damping forces.
These and other objects of the invention are accomplished by an apparatus for variably damping motion using an MR fluid. The apparatus includes a housing for containing a volume of MR fluid; a piston adapted for movement within the housing, the piston being formed of ferrous metal, having a number, N, of the windings of conductive wire incorporated therein to define a coil that produces magnetic flux in and around the piston; and having a configuration in which ##EQU1## where A.sub.core is a minimum lateral cross-sectional area of the piston within the coil, A.sub.path is a minimum lateral cross-sectional area of magnetically permeable material defining a return path for the magnetic flux, A.sub.pole is the surface area of the piston's magnetic pole, B.sub.opt is an optimum magnetic flux density for the MR fluid, and B.sub.knee is a magnetic flux density at which the ferrous metal begins to saturate.
The housing may be provided with a sleeve of ferrous material to increase the cross-sectional area of the return flow path for the magnetic flux, A.sub.path, for configurations in which the return path for the magnetic flux is through the housing. Alternatively, the housing may be a twin-tube design; the magnet may be formed on a spool-shaped piston or wound as a toroid thereon; the magnet could be positioned within the twin-tube housing rather than on the piston; loss of MR fluid can be prevented by topping the damper with a less dense fluid, using a scraper and seal combination, or using a sealless design. The piston may be formed from conventional ferrous materials (in either solid or laminate form) or from powdered metals. These features may be embodied in a mount as well as in a damper.
Other features, advantages and characteristics of the present invention will become apparent after a reading of the following detailed description.